Update LESSONPLAN.md

Author: Tjebbee

Week2/LESSONPLAN.md

@@ -24,19 +24,54 @@ Notes:
 - Asynchronous helps us do multiple things in parallel
 
 
-## 3. What callbacks are and how to write your own
+## 3. Callbacks
+
 ### Explanation
+
+A callback in JavaScript is basically a function(callback) being passed as a parameter to another function which after some point of time would execute the function passed or invoke the callback.
+
+Callbacks were primarily introduced in JavaScript to achieve asynchronous behaviour 
+(https://codeburst.io/javascript-what-the-heck-is-a-callback-aba4da2deced)
 ### Example
+Consider a situation where person A wishes to go out for a movie with a friend person B one evening. Person A finds out the time and place and now needs to share it with B. A picks up the phone and tries to call B. Let's say that B is currently busy with some work and can't answer the phone. Person A has now got two options. One option is to stay on the line until B picks up the phone and then share the movie details. Or A could drop a voicemail and ask B to __callback__ once free.
+
+``` javascript
+function doHomework(subject, callback) {
+  alert(`Starting my ${subject} homework.`);
+  callback();
+}
+function alertFinished(){
+  alert('Finished my homework');
+}
+doHomework('math', alertFinished);
+```
+
 ### Exercise
-### Essence
-Notes:
 
-- Callbacks are a way to introduce asynchronocity
+#### 1. What will happen? 
+#### 2. How to turn the output order around?
+``` javascript
+function first(){
+  // Simulate a code delay
+  setTimeout( function(){
+    console.log(1);
+  }, 500 );
+}
+function second(){
+  console.log(2);
+}
+
+first();
+second();
+```
 
 
+### Essence
+you can’t just call one function after another and hope they execute in the right order. Callbacks are a way to make sure certain code doesn’t execute until other code has already finished execution.
+
 SECOND HALF (14.00 - 16.00)
 
-## 4. How the event loop works
+## 4. Event loops
 
 ### Explanation
 ### Example
@@ -48,7 +83,7 @@ Notes:
 - It determines when any given function is executed
 
 
-## 5. Show 3 commonly used array functions (filter, reduce, map)
+## 5. 3 commonly used array functions (filter, reduce, map)
 ### Explanation
 ### Example
 ### Exercise